1. Field of the Invention
The present invention relates to fuel cell stacks and methods for making the fuel cell stacks. In particular, the present invention relates to a fuel cell stack having a separator made from a metal plate and a method for making such a fuel cell stack.
2. Description of the Related Art
A polymer electrolyte membrane (PEM) fuel cell includes a solid polymer electrolyte membrane having proton conductivity, and a pair of electrodes (anode and cathode) sandwiching the electrolyte membrane. Fuel gas is supplied to the anode while oxidizing gas is supplied to the cathode so that electric power is generated as a result of an electrochemical reaction that occurs on the surfaces of the electrolyte membrane.
One fuel cell includes a membrane electrode assembly (MEA) having a pair of electrode catalyst layers and an electrolyte membrane interposed between the electrode catalyst layers, and two separators respectively disposed at two sides of the MEA to define gas channels.
Each separator that defines a gas channel for the fuel gas or oxidizing gas also defines a temperature-controlling medium channel through which a temperature-controlling medium flows.
Since the separator also functions as a conduction path for retrieving output from the electrode catalyst layer of each fuel cell, the separator is entirely composed of an electrically conductive material.
A fuel cell stack includes as many fuel cells as needed to derive a required output voltage, the fuel cells being stacked in series, and a pair of current collectors respectively disposed at the two ends of the stack in the stacking direction. Electric power output from each fuel cell is derived from the current collectors.
According to related art, separators for fuel cells have been made of carbon, which has both corrosion resistance and electrical conductivity. In recent years, metal separators are used to reduce the cost of fuel cells and decrease the thickness of the separator while increasing the output density.
Examples of existing metal separators include a stainless steel plate with titanium or a titanium alloy clad thereon serving as a corrosion-resistant coating, a stainless steel plate having thereon a noble metal plating layer serving as a corrosion-resistant coating, a stainless steel plate having thereon a titanium coating layer serving as a corrosion-resistant coating, and a metal plate having thereon a noble metal plating layer serving as a corrosion-resistant coating. Japanese Unexamined Patent Application Publication Nos. 2005-002411, 2005-190968, 2005-276807, and 2005-317479 disclose such separators.